Nondestructive inspection systems and medical imaging apparatuses using a transmitted X-ray are conventionally known. For example, the nondestructive inspection system disclosed in Patent Document 1 measures the thickness of a target object located on or within a base member and made of a material different from that of the base member. In this system, the target object is irradiated with an X-ray whose wavelength is selected so that the peak of the entire spectrum of the X-ray is located near the absorption edge of the element of the target object, and the thickness of the target object is determined based on the intensity of a transmitted X-ray at the aforementioned wavelength near the absorption edge. More specifically, the intensity of the X-ray that has passed through the base member and target object is measured near the high-energy (short-wavelength) side of the absorption edge and also near the low-energy (long-wavelength) side of the absorption edge, and the thickness of the target object is calculated from the intensity values, the linear absorption coefficient of the target object near the high-energy side of the absorption edge and the linear absorption coefficient of the target object near the low-energy side of the absorption edge.
This conventional transmitted X-ray measurement method is basically designed on the premise that the target object is made of one kind of a known constituent (or element). Such a method is suited for some specific purposes, such as measuring the thickness of a specific heavy metal adhered to the inside of a pipe, closed container or similar structure. However, the method cannot be used if the target object is a mixture of two or more elements and/or compounds. Although Patent Document 1 also discloses a method for determining the content percentage of a specific element contained in an alloy object, the method only yields the content percentage of one specific element; it provides no information about the content percentage of the other elements. Another problem results from the use of a formula that utilizes the linear absorption coefficient, which represents the attenuation rate of an X-ray per unit length of material. Calculating the content percentage of a specific element by this formula leads to a rather inaccurate result since the formula does not take into account the influence of elements other than the specific one.
Currently, many people working in the industrial, medical or other fields have a strong demand for investigating the stereographic or three-dimensional distribution of elements or compounds contained in a target object. One of the conventionally known systems for obtaining a three-dimensional image by means of a transmitted X-ray is computer tomography (CT). These systems reproduce a three-dimensional image by illustrating the difference in the density of a substance inside a target object by shading or color variations. However, none of those systems can measure the three-dimensional concentration distribution of an element or compound contained in the target object.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H11-287643